Beirut: The Islamic State group has captured more than a dozen villages and hills during a fresh offensive in norther Syria, opposition activists said on Saturday.

The IS territorial gains bring the extremist group close to the main highway that links the capital, Damascus, with the country's largest city of Aleppo.

IS also clashed with rival insurgent groups near the border with Turkey where they have been on the offensive for days, forcing tens of thousands of residents to flee toward safer areas near Turkey.

The Britain-based Syrian Observatory for Human Rights said the fighting between IS and government forces is concentrated in areas east of the town of Khanaser, which has changed hands several times in recent months.

An activist based in Aleppo told The Associated Press via Skype that IS launched its latest offensive in the area a day earlier and by today was in control of some 18 small villages.

The activist spoke on condition of anonymity for fear of reprisals from IS.

Khanaser is strategic since it's on the highway that links government-held parts of Aleppo with the rest of the country. The extremist group has cut the highway several times in recent months but government counteroffensives were able to push them back.

The Observatory said traffic on the highway was interrupted by IS shelling of the area today before resuming as usual.

To the north, IS continued in its offensive near the Turkish border capturing the village of Tal Shaaer from opposition fighters.

"Daesh is advancing against the regime and the rebels at the same time," said the Aleppo-based activist.

Around 30,000 displaced persons have fled from their shelters near the Turkish border, according to the New York-based Human Rights Watch.

Medicins Sans Frontieres, also known as MSF or Doctors Without Borders, said it was "extremely worried" about the displaced people's security and access to health care.

The international medical relief organization said it knew of just five operating hospitals in the Azaz district, where much of the fighting is concentrated.

The violence came as Syrian government and opposition delegations are taking part in indirect peace talks in Geneva aiming to end the country's five-year civil war that has killed more than 250,000 and displaced half of Syria's population.

Near the city of Aleppo where government forces have been on the offensive for days, the situation was relatively calm Saturday after rebels captured some areas back from government forces, the Aleppo-based activist said.